Ultrasonic motor

ABSTRACT

This invention provides an ultrasonic motor in which a central transducer means is sandwiched between front and rear stubs. The front stub is made of either magnesium or magnesium alloy, and there is provided a cylindrical housing having means for clamping the stubs axially together. The clamping locations on the stubs are adjacent the transducer and the configuration of the connection between the housing and the front stub includes a frusto-conical bearing face, the latter being necessary due to the weakness of the material from which the front stub is constructed. It is highly desirable that the front stub be made of magnesium or magnesium alloy, however, because of the suitability of such material for impedance matching into water or liquid.

I United States Patent 1191 1111 3,845,332

Last 1 Oct. 29, 1974 [54] ULTRASONIC MOTOR 3.368.085 2/1968 McMaster et a1 310/82 x 3,370,186 2/1968 A t h 1 310/82 [75] Inventor: Lash 3,578,996 5/1971 310/87 ana a [73] Assignee: Ontario Research Foundation, Primary Emmi'lerMafk Budd Ontario, C n d Attorney, Agent, or FirmSim & McBurney [22] Filed: Mar. 14, 1972 [57] ABSTRACT 1 1 pp Noi 2349612 This invention provides an ultrasonic motor in which a Related Applitafion Dam central transducer means is sandwiched between front and rear stubs. The front stub is made of either mag- {631 gy xr gg of H3298 nesium or magnesium alloy, and there is provided a cylindrical housing having means for clamping the [52] U S Cl 310/8 7 310/8 2 310/8 3 stubs axially together. The clamping locations on the 151 Int Cl$122131: .1... 110111 17/00 Stubs are adiacem the transducer and cmflgura' [58] Field of Search 310/82, 8.3, 8.7, 9.1, COMM and front stub includes a frusto-comcal beanng face, the 310/94 latter bemg necessary due to the weakness of the ma- 1 from which the front stub is constructed. It is [56] References Cited tefna highly des1rable that the front stub be made of magne- UNITED STATES PATENTS sium or magnesium alloy, however, because of the suitability of such material for impedance matching carpa 3,230,403 1/1966 Lewis et a1 310/87 mm water or 3,283.182 11/1966 Jones et a1 310/87 10 Claims, 8 Drawing Figures PAIENIEUIHZB m4 $845332 L J INVENIOR:

ANTHONY Q. LAST PATENT AGENTS PAIENTEMmsnu 3.:3455332 l N VEN '1 ()R. Ann-mm LAET BY SM PATENT AGE NTS ULTRASONIC MOTOR This application is a continuation-impart of U.S. application Ser. No. 113,298, filed Feb. 8, I971 under the same title now abandoned.

This invention relates generally to devices capable of generating high-frequency mechanical vibrations.

GENERAL BACKGROUND OF THE INVENTION The primary method of producing high-frequency mechanical vibrations, particularly those in the ultrasonic range, is to utilize the properties of certain specialized materials to transform energy in an electric or magnetic form into energy of a mechanical vibratory form. Such devices are commonly termed transducer, and those skilled in this art will be familiar with their various forms. The following patents may be referred to to gain a general picture of developments in this and related fields: US. Pat. No. 2,6U6,l74, Aug. 5, I952, Kolthoff et al', US. Pat. No. 3,l65,299, Jan. l2, I965, Balamuth et al; German Pat. No. 712,216, Aug. 14, I941. Hertz et al; German Pat. No. 716,231, Jan. 15, I942, Hertz et al', German Pat. No. 960,893, Mar. 13, I957, Hertz; German Pat. No. 7l8,744, Mar. 19, I942, Schofer; German Pat, No. 994,667, June 21, I956, Sauter.

Essentially, there are two main types of transducers, known as electrostrictive and magnetostrictive. The present invention is particularly adapted to utilize the electrostrictive type of transducer. Of the two types of electrostrictive effects known as piezoelectric" and ferroelectricf it is the former effect which is preferably used in the electrostrictive application ofthis invention. The piezoelectric" effect is a phenomenon exhibited by materials such as quartz, Rochelle salt, and tourmaline, wherein the crystal changes its length over certain crystallographic axes by a differential varying directly with an electric field placed across the crystal. Thus, a high-frequency alternating electric field placed across the appropriate axis of the crystal of quartz causes the crystal to vibrate at the same frequency as that of the electric field. Generally, natural crystals have been replaced in modern practice by artificial crystals, and such ceramic materials as barium titanate and lead zirconate titanate can be made to act in 21 piezoelectric manner by prepolarization.

In the adaptation of the above effects to accomplish the objects ofthe present invention, a sandwich" con struction is utilized throughout, wherein two metal stubs sandwich between them a transducer assembly comprising a central electrode and two transducer "halves." This construction permits a selection of the resonance frequency at which the efficiencies of liquid phase processing in the cavitation range are highest.

OBJECTS OF THE INVENTION In the particular application of the transducer vibration-generating principle to the problem ofliquid phase processing, it is of great advantage for the total assembly to be small, compact, and easily manipulated manually. This is true in laboratory work, where speed and ease of operation are advantageous.

Accordingly, it is one object ofthis invention to pro vide an ultrasonic generating assembly which is small. compact and capable of manual control and manipulation.

It will be appreciated, however. that rendering the ultrasonic generator into a small, compact and manu- III fill

ally controllable form carries with it the risk, particu larly for the higher frequencies and energy outputs, that adequate cooling of the parts in which heat accumulates will not take place, resulting in over-heating, thermal stress, and the danger of breakdown.

Accordingly, it is a further object of this invention to provide an ultrasonic motor construction which is designed to reduce heat generation to a minimum.

It is also a further object of the invention to use magnesium or magnesium alloy as one part of the metal sandwich for the improvement of impedance matching between the ceramic and the liquid phase, and to provide a construction in which the risk of fatigue or thread failures in the magnesium is reduced to a minimum.

Generally, the above objects are attained by providing an ultrasonic motor which eliminates the use of the centrally or peripherally bolted construction which is conventionally used in the design of similar transducers. Instead, the pressure is applied to the transducer elements at points spaced by one-fourth wavelength or less by means ofa cylindrical housing and retaining ring capable of compressing both transducer elements between the metallic sandwich. The bearing surface of the housing and magnesium metal front stub takes the form of a frusto-conical surface, defining an angle of about 45 with the central axis of the stub. The reentrant point where the conical part of the magnesium or magnesium alloy joins the cylindrical part is relieved by a small radius and polished to eliminate scratches which may be a potential site for fatigue initiation.

Accordingly, this invention provides an ultrasonic motor comprising transducer means, a rear stub in surface contact with said transducer means, a front stub in surface contact with said transducer means, and being made ofa material chosen from the group: magnesium, magnesium alloy; means for exciting the transducer means, a substantially cylindrical housing having means for clamping said front and rear stubs axially together, the clamping locations on said stubs being adjacent the transducer means so that a major portion of each stub extends axially beyond its clamping location in the direction away from the transducer means, the front stub having an outwardly and rearwardly flaring flange with a substantially frusto-conical bearing face, the cylindrical housing having a frusto-conical bearing surface complementary to said bearing face adapted to grip the front stub through the said bearing face.

DESCRIPTION OF DRAWINGS Two embodiments of this invention are shown in the accompanying drawings, in which like numerals denote like parts through out the several view, and in which:

FIG. I is a perspective view of the first embodiment of an ultrasonic motor constructed in accordance with this invention;

FIG. 2 is an axial sectional view of the ultrasonic motor shown in FIG. 1;

FIG. 3 is an axial sectional view of the upper portion of the second embodiment of this invention;

FIG. 4 is an exploded perspective view of the essential components of the first embodiment of the ultrasonic motor shown in FIG. I, and

FIGS. 5, 6, 7 and 8 illustrate fatique failure in magnesium stub designs tested and rejected during the development of the ultrasonic motor of this invention.

In FIG. 1, an ultrasonic motor consists of a halfwave concentrator horn 12 preferably of a titanium alloy (90 Ti 6 Al 4 V), a quartenwave magnesium alloy front stub 14, two handles 16 and 17 attached to a handle ring 18, a cylindrical housing 20, a quarterwave rear stub 22, preferably of steel, a retaining ring 24 which cooperates with the cylindrical housing 20 to urge the stubs l4 and 22 together, and a terminal assembly 26.

The theory and construction of the titanium alloy half-wave concentrator horn 12 per se form no part of this invention. The reduction in diameter from the top of the bottom of the horn 12 is of course to produce a concentration of the vibrational energy, and the profile of the transition may define a curved surface which is of circular, catenoidal, exponential, or otherwise suitable shape. The principles involved in the design of an exponential horn are set out in an article by L. G. Merltulov and A. V. Kharitonou, Theory and Analysis of Sectional Concentrators" Soviet Physics Acoustics 5 1959, page l88. Tests have shown an exponential transition to be superior to a circular one, from the point of view of fatigue failures at full power.

The titanium concentration horn is utilized to transmit the vibration directly to the liquid because magnesium tends to erode very rapidly by cavitation in water.

The theory behind the use of a quarter-wave magnesium stub between the transducer and the titanium con centrator born 12 involves the question of impedance matching, which is fully discussed in my US. Pat. No. 3,524,083 Device for the Concentration of Vibrational Energy, issued Aug. ll, l970, and will be alluded to later in this disclosure.

In this compact size of transducer, the magnesium alloy will not accept any form of thread for tightening the transducer sandwich owing to (a) creep in the threads and (b) fatigue cracking at a point where the bolt thread ceases to exert pressure in the magnesium. This does not apply to the attachment of the half-wave horn, however, since the pressure required is much less. The only satisfactory method thus far found for attaching the magnesium alloy stub to the front stub and compressing the transducer elements in between, is by means ofa frusto-conical shape on the magnesium stub mating with the housing.

Referring now specifically to FIGS. 2 and 4, there is provided a flat cooling disc 28 having a plurality of radial slots 30 which extend from the periphery 31 of the cooling disc 28 inwardly to locations short ofthe center of the cooling disc 28.

Directly above the cooling disc 28 is a first circular plate-like shim 32 which has a central aperture 34 of which the diameter is large enough to leave uncovered the inner extremities of the slots 30. The first shim 32 is slightly larger than the cooling disc 28.

Directly below the cooling disc 28 is a second circular plate-like shim 36 the same size as the first shim 32.

Directly above and in contact with the first shim 32 is a first transducer element 37 which has a central opening 38 in registry with the central aperture 34 of the first shim 32.

A locating ring 40 in the shape of a short, flanged cylinder is adapted to fit snugly within the central opening 38 with its flange 4| resting against the first transducer element 37.

Above and in surface contact with the first transducer element 37 is the quarter-wave rear stub 22 having an open-ended passage 44. The rear stub 22 is substantially cylindrical in shape, and the open-ended pas' sage 44 is coaxially located within the rear stub 22. At the lower end of the passage 44, the rear stub 22 is re cessed to snugly accommodate the flange 4i of the locating ring 40. The rear stub 22 has, at its lower end, an outwardly projecting ledge 46, and the retaining ring 24 is adapted to snugly embrace the rear stub 22, sliding down to abut the ledge 46. As can be seen in PK]. 2, the retaining ring 24 has a flange 47 with notches 48 at points spaced l20 apart, and further has external threads 50 for engagement with the cylindrical housing 20.

Directly below and in surface contact with the second shim 36 is a second transducer element 52, having a central opening 54 ofthe same diameter as the opening 38 in the first transducer element 37. A locating cylinder 56 is adapted to fit snugly within the central opening 54. Directly beneath and in surface contact with the second transducer element 52 is the quarterwave magnesium stub 14. The quarter-wave stub 14 is essentially cylindrical. but has at its upper end an outward flange 57 which has a cylindrical surface 58 and a frusto-conical surface 59, preferably defining an angle of about 45 with the central axis of the quarterwave stub 14. The quarter-wave stub 14 has a central upper cylindrical recess 61, intended snugly to accommodate the locating cylinder 56, as shown in FIG. 2.

The titanium alloy concentrator horn l2 has an integral threaded shaft 62, intended to be received in a threaded bore 64 extending upwardly from the bottom of the quarter-wave stub l4 and coaxial therewith.

Naturally, the mating faces of all of the elements 22, 37, 32, 30, 52, 14 and 12 are planar.

The cylindrical housing 20, at its bottom end, has an inward step 66 and is configured to define a cylindrical surface 67, adapted snugly to embrace the outer surface 68 of the quarter-wave stub 14, and a frustoconical bearing face 69 which has the same cone angle as the frusto-conical surface 59 and is intended to mate therewith in surface'to-surface contact. Thus. the surface 68 and bearing face 69 provide for the cylindrical housing 20 a secure grip on the upper end of the quarter-wave stub 14.

As can be seen particularly in FIG. 2, the cylindrical housing encloses. and in part defines, an annular chamber 70 outwardly adjacent the cooling disc 28. The cylindrical housing 20 includes aperture means, consisting in H6. 2 of a bore hole 72, by means of which the annular chamber 70 communicates with the exterior of the cylindrical housing 20. Although only one bore hole 72 is shown in H05. 1, 2 and 4, obviously more than one can be provided, if desired. The cylindrical housing 20 also has two antipodal threaded bores 74, which are adapted to receive locating bolts 75, the purpose for which will now be explained.

The handle ring 18 shown in FIG. 1 can be seen in FIG, 2 to be dimensioned so as to embrace the cylindrical housing 20 and is cushioned by two O-rings (not shown). It is intended that the handle ring 18 be located axially with respect to the cylindrical housing 20 in a position abutting the heads of the locating bolts 75. This has been shown in FlG. 2.

As shown in FIG. I, the handle 17 has a vertical extension 76 which is adapted to be bolted to the handle ring 18 by means of countersunk bolts 78.

Handle 16 is bolted radially into the handle ring 18 by conventional means now shown. Handle 16 can be provided with an axial bore of about 0.6 inches diameter to permit the insertion of a one-half inch rod, whereby the entire probe may be supported from a retort stand and is then capable of being axially rotated with handle 16 acting as the axis of revolution. By providing both the handles in such a way that they form an angle less than 180 (in this case about 135),a greater degree of manual control is available than if the handles were substantially coaxial.

In the first embodiment shown in FIGS. 1, 2 and 4, the terminal assembly 26 bolted to the upper end of the quarterwave rear stub 22 includes a top-hat housing 79 having an attachment flange 81 and a part-cylindrical crown 82, and a terminal element 84. The partcylindrical crown 83 has a portion removed to define a vent opening 86, and has a circular aperture 87 in its top adapted to receive the terminal element 84. Suitable bolts 88, openings 89 and threaded bores 90 are provided for securing the terminal element 84 to the top of the part-cylindrical crown 83 and for securing the attaching flange 81 against the rear stub 22.

In the first embodiment shown in FIG. 2, the ultrasonic motor when held vertically as shown in FIG. I is such that the vent opening 86 is elevated above the bore hole 72, so that the generation of heat in the transducer elements 37 and 52 will cause convection currents of air to enter the annular chamber 70 through the bore hole 72, pass through the slots 30 and into the open-ended passage 44 thence back out into the atmosphere through the vent opening 86.

In FIG. I, a cable 92 and a female terminal 93 are shown in broken lines connecting with the terminal element 84.

As shown in FIG. 2, a lead from the terminal element 84 is connected by an insulated wire 94 whose end 95 proceeds through the center of the cooling disc 28 to the center of the shim 36 it is soldered in place. The terminal element 84 is grounded through the top-hat housing 79 to the rear stub 22, and thus to the front stub 14 by virtue of the electrical connection between the stubs constituted by the cylindrical housing 20.

As shown in FIG. 4, the cylindrical housing has faceted surfaces 96 (only one shown in FIG. 4) by which the cylindrical housing can be gripped while the retaining ring 24 is rotated.

Turning now to FIG. 3, the second embodiment of this invention will be described. In FIG. 3, the terminal assembly 26' differs from the terminal assembly 26 in that the vent opening 86 is not present. The terminal element 84 is unchanged. although it is shifted slightly to one side of center in the tophat housing 79'. The seal between the attachment flange 8| and the upper end ofthe rear stub 22 is substantially air-tight. as is the seal between the flange 97 of the terminal element 84 and the part-cylindrical crown 83'. Opening through the top of the part-cylindrical crown 83' is an air-line connection 98 to which an air pressure line shown in broken lines at 99 may be attached. Air introduced under pressure through the air-line 99 thus passes into the open-ended passage 44, through the slots 30 shown in FIGS. 2 and 4 into the annular chamber 70, and thence into the atmosphere through the bore holes 72.

As shown in FIG. I, the handle ring 18 is constructed in two parts, and is adapted to be bolted together in conventional fashion by bolts 100.

THEORETICAL CONSIDERATIONS The following is a summary of the theoretical considerations entering into the design of the ultrasonic motor disclosed herein, and is intended to clarify certain points already mentioned.

Firstly, it is to be stressed that the ultrasonic motor disclosed herein is especially designed for liquid loads. It is also important to realize that, for ease of handling and manoeuvrability, it is highly desirable to keep the stubs, transmission horn, etc., down to the shortest possible length. The minimum length under these circumstances is the sum of one half wave-length for the motor" (front and rear stubs), plus the shortest possible concentration horn for actually transmitting the vibration to the water or other liquid.

A brief digression into theory is necessary to show why magnesium is the best available material to constitute the front stub. In order to obtain the highest possible efficiency, it is necessary to use a good impedance match" between the electrostrictive ceramic and the liquid load. By increasing the efficiency, this design cuts down the amount of heat generated in the motor. Reference is now made to US. Pat. No. 3,524,083. issued Aug. ll, 1970, inventor, Anthony J. Last, entitled Device for Concentration of Vibrational Energy. In column 7 of that patent it is indicated that Zm the impedance of the stub, should be ideally V Z X Z,. where 2 is the impedance of the liquid and Z is the impedance of the ceramic. For water an PZT ceramic, the impedances are 1.48 X l0 kg/m s and 30 X It) Kg/rn s respectively. The ideal impedance for a quarter-wave stub is therefore v X L48 X It): 6.66 X l0 Kglm 's. Magnesium has an impedance of 9.9 X It) Kg/m s and the next closest is aluminum at 17.3 Kg/m' s.

Although it would be ideal to use the magnesium stubs to transmit the vibrational energy directly into water, this cannot be practically carried out because magnesium tends to erode by cavitation very rapidly in water. For this reason, a half-wave horn acting as a transmission line is used between the magnesium and the water. Ifsuch a transmission line does not alter the velocity of sound through it, then it is not seen" by the load, which only sees" the combination of magnesium and PZT ceramic impedances. Preferably, this halfwave horn is made of an erosion resistant material such as titanium.

While magnesium is the best available material for the front stub, for reasons explained above having to do with impedance matching, magnesium is nonethless weaker than steel and aluminum, and fatigue failure becomes a considerable problem in sandwich constructions such as that disclosed in US. Pat. No. 3,l0l ,4 l9, issued Aug. 20, I963 to S. R. Rich and entitled Electromechanical Transducer System, and in US Pat. No. 3,140,859, issued July 14, 1954 to T. I. Scarpa, and entitled Electroacoustic Sandwich Transducers." In FIG. I ofthe Rich patent and FIGS. 1, 2 and 3 of the Scarpa patent, the fatigue failures always tend to occur in the magnesium at the point where the bolt thread ends. Reference is now made to FIG. 5 of the attached drawings, in which the shank Ill] of a bolt is threaded into a block 112 of magnesium. Tension has been placed upon the shank 110 while the block 112 is an chored, and the result is a radial fatigue failure 114 at about the location where the bolt thread ends. Actual tests carried out during the development of the ultrasonic motor disclosed herein have clearly shown the nature of this radial fatigue failure.

Also during the development of the ultrasonic motor disclosed herein, the arrangement illustrated in FIGS. 6, 7 and 8 were tried unsuccessfully. In each case, fatigue failures I14 tended to occur readily because of the low strength of magnesium.

Of all the configurations tested during the development of the ultrasonic motor disclosed herein, only that shown in FIGS. 2 and 4 of the drawings demonstrated a high enough resistance to fatigue failure to be useful in the construction of the ultrasonic motor. To recapitulate, this construction involves a frusto-conical surface 59 on the flange 57 at the rear portion of the magnesium front stub 14 and a complementary or mating frusto-conical bearing face 69 at the lower end of the cylindrical housing 20. While not wishing to be bound by theory, it is suspected that the high resistance of the FIG. 2 configuration to fatigue failure results from the fact that the force exerted by the cylindrical housing 20 upon the front stub 14 has a radially inward component which tends to compress the rear portion of the stub 14 radially inwardly, thus counteracting a tendency for fatigue failures to develop in the longitudinal direction.

Because of its strength characteristics, it is preferred that the cylindrical housing 20 be made from steel or other particularly strong alloy. However, in order to be able to manufacture the cylindrical housing 20 from steel. it is necessary that the location at which the stubs (particularly the front stub 14) are gripped by the cylindrical housing be as close as possible to the ceramic transducer, so that the projecting portion of the stub is free to vibrate. If the cylindrical housing were to clamp the magnesium front stub 14 at its front portion (the end opposite from the end in contact with the transducer), the efficiency ofthe ultrasonic motor would be low because the damping of the steel cylindrical hous ing against the stub would adversely affect the impedance of the magnesium and the matching characteristic would, to a large extent, be lost. As before mentioned, it is preferred that the distance between the clamping locations on the stubs be not more than one-fourth wavelength, and preferably less. The composite element which includes the cooling disc 28 and the shims 32 and 36 defines the location of a node in the vibrational system. while the end of the stubs remote from the ceramic transducer constitute antinodes. Table I below shows the effect of antinode clamping, as opposed to nodal clamping of the kind utilised in the ul trasonic motor disclosed herein. Antinode clamping is disclosed in US. Pat. No. 3,283,182 issued Nov. I, 1966 to J. B. Jones et al., and entitled Transducer As sembly."

TABLE l-Continucd Transducer Frequency Generator Impedance Best Power Type H7. Output of Input to Amps Generator Transducer Ohms Watts Nodal 28.67 .3 2000 50.0 Antinode 28.47 .4 2000 66.0 Nodal 28.44 .4 2000 95.0 Antinode 28.40 .5 2000 90.0 Nodal 23.38 .5 2000 I400 Antinode 29.76 .2 I000 I4.0 Nodal 27.77 .2 I000 64.0 Antinode 28.36 .3 I000 36.0 Nodal 2813 .3 I000 75.0 Anlinude 28.13 .4 I000 76.0 Nodal Ill .02 .4 I000 I 20.0 Anlinotle 2B 34 .5 1000 I401] Nodal .5 I000 I500 TABLE I Table 1 shows the power input to the transducer in watts at various power level settings of the generator at impedance outputs of the generator of 2,000 and l,000 Ohms. It will be noted that the power input to the nodal type is consistently better than that of the antinode type into a water load.

Table 1 clearly indicates that the quarter wave stubs, particularly the magnesium front stub 14, should be clamped as close as possible to the ceramic transducers.

What I claim is:

I. An ultrasonic motor comprising:

transducer means,

a rear stub in surface contact with said transducer means,

a front stub in surface contact with said transducer means, and being made of a material chosen from the group consisting of magnesium and magnesium alloy,

means for exciting the transducer means such that the ultrasonic motor vibrates in its fundamental mode,

a substantially cylindrical housing having means for clamping said front and rear stubs axially together, the clamping locations on said stubs being immediately adjacent the transducer means and spaced from an anti-node so that a major portion of each stub extends axially beyond its clamping location in the direction away from the transducer means,

the front stub having an outwardly and rearwardly flaring flange with a substantially frusto-conical bearing face, the cylindrical housing having a frusto'conical bearing surface complementary to said bearing face adapted to grip the front stub through said bearing face.

2. The invention claimed in claim 1, in which the frusto-conical bearing face defines an angle of substantially 45 with the axis of the front stub.

3. The invention claimed in claim 1, in which the front and rear stubs are substantially cylindrical.

4. The invention claimed in claim 1, in which the distance between said clamping locations is not greater than one-fourth wavelength.

5. The invention claimed in claim I, which further includes a cooling element, the transducer means including a first and a second transducer element sandwiching the cooling element between them, the first transducer element having a central opening, the rear stub having a passage in registry with said central open- 9 ing, and the cooling element having passageway means for communicating said central opening in the first transducer element with the space surrounding the cooling element, whereby a fluid cooling medium passing through said passageway means may absorb heat by conduction from the said transducer elements.

6. An ultrasonic motor as claimed in claim 5, in which the cooling element includes a fiat cooling member having mutually convergent slots extending from the periphery to locations short of a hypothetical point of convergence, and two plate-like shims, one on either side of the flat cooling member and each contacting a stub, the shim contacting the rear stub having a central aperture of sufficient size to leave uncovered the inner extremities of the slots.

7. The invention claimed in claim 6, in which said cylindrical housing encloses and in part defines an annular chamber outwardly adjacent said cooling disc, said slots being in communication with the annular chamber, the cylindrical housing having aperture means by which the annular chamber communicates with the exterior of the cylindrical housing.

8. An ultrasonic motor as claimed in claim 7, in which the transducer elements are piezoelectric crystals of a material chosen from the group consisting of: barium titanate, and lead zirconate titanate or like high power material; and in which for each transducer element the cooling disc and the shims constitute one electrode, while the respective stub constitutes the ill other electrode, the two stubs being in electrical communication through the cylindrical housing, the latter being of an electrically conductive material,

the rear stub having an open-ended passage of which one end is in registry with said central aperture,

the ultrasonic motor further comprising a terminal assembly fixed to the rear stub at the other end of said openended passage and including a double terminal,

an insulated wire constituting one lead from the terminal and extending along said open-ended passage to connect to one shim, the terminal being grounded to the rear stub.

9. An ultrasonic motor as claimed in claim 8, further including means defining an air-line connection communicating with said passage, whereby an air forced under pressure into said air-line connection passes through said passage and said slots into said annular chamber, emerging therefrom through said aperture means.

10. An ultrasonic motor as claimed in claim 8, in which the terminal assembly includes a vent opening through which said passage is open to the surround, whereby use of the ultrasonic motor in an attitude in which the vent opening is elevated above the aperture means causes air to circulate by convection from the annular chamber through the slots and the passage to said vent opening, 

1. An ultrasonic motor comprising: transducer means, a rear stub in surface contact with said transducer means, a front stub in surface contact with said transducer means, and being made of a material chosen from the group consisting of magnesium and magnesium alloy, means for exciting the transducer means such that the ultrasonic motor vibrates in its fundamental mode, a substantially cylindrical housing having means for clamping said front and rear stubs axially together, the clamping locations on said stubs being immediately adjacent the transducer means and spaced from an anti-node so that a major portion of each stub extends axially beyond its clamping location in the direction away from the transducer means, the front stub having an outwardly and rearwardly flaring flange with a substantially frusto-conical bearing face, the cylindrical housing having a frusto-conical bearing surface complementary to said bearing face adapted to grip the front stub through said bearing face.
 2. The invention claimed in claim 1, in which the frusto-conical bearing face defines an angle of substantially 45* with the axis of the front stub.
 3. The invention claimed in claim 1, in which the front and rear stubs are substantially cylindrical.
 4. The invention claimed in claim 1, in which the distance between said clamping locations is not greater than one-fourth wavelength.
 5. The invention claimed in claim 1, which further includes a cooling element, the transducer means including a first and a second transducer element sandwiching the cooling element between them, the first transducer element having a central opening, the rear stub having a passage in registry with said central opening, and the cooling element having passageway means for communicating said central opening in the first transducer element with the space surrounding the cooling element, whereby a fluid cooling medium passing through said passageway means may absorb heat by conduction from the said transducer elements.
 6. An ultrasonic motor as claimed in claim 5, in which the cooling element includes a flat cooling member having mutually convergent slots extending from the periphery to locations short of a hypothetical point of convergence, and two plate-like shims, one on either side of the flat cooling member and each contacting a stub, the shim contacting the rear stub having a central aperture of sufficient size to leave uncovered the inner extremities of the slots.
 7. The invention claimed in claim 6, in which said cylindrical housing encloses and in part defines an annular chamber outwardly adjacent said cooling disc, said slots being in communication with the annular chamber, the cylindrical housing having aperture means by which the annular chamber communicates with the exterior of the cylindrical housing.
 8. An ultrasonic motor as claimed in claim 7, in which the transducer elements are piezoelectric crystals of a material chosen from the group consisting of: barium titanate, and lead zirconate titanate or like high power material; and in which for each transducer element the cooling disc and the shims constitute one electrode, while the respective stub constitutes the other electrode, the two stubs being in electrical communication through the cylindrical housing, the latter being of an electrically conductive material, the rear stub having an open-ended passage of which one end is in registry with said central aperture, the ultrasonic motor further comprising a terminal assembly fixed to the rear stub at the other end of said openended passage and including a double terminal, an insulated wire constituting one lead from the terminal and extending along said open-ended passage to connect to one shim, the terminal being grounded to the rear stub.
 9. An ultrasonic motor as claimed in claim 8, further including means defining an air-line connection communicating with said passage, whereby an air forced under pressure into said air-line connection passes through said passage and said slots into said annular chamber, emerging therefrom through said aperture means.
 10. An ultrasonic motor as claimed in claim 8, in which the terminal assembly includes a vent opening through which said passage is open to the surround, whereby use of the ultrasonic motor in an attitude in which the vent opening is elevated above the aperture means causes air to circulate by convection from the annular chamber through the slots and the passage to said vent opening. 